[[1]}]
A class \({C} \)  of \(\Sigma \) -structures
is monadically stable (monadically NIP) if and only if for every monadic
extension \(\Sigma ^+\)  of \(\Sigma \)  every \(\Sigma ^+\) -formula \(\varphi (x,y)\) 
is stable (NIP) over \({C} [\Sigma ^+]\) .
